Terrestrial solar array including a rigid support frame

ABSTRACT

A concentrator photovoltaic solar cell array system includes a central support mountable on a surface and a solar cell array including triple junction III-V semiconductor compound solar cell receivers and a support frame coupled to the solar cell array and carried by, and rotatable with respect to, the central support about an axis orthogonal to the central longitudinal axis. The support frame can include (i) a first frame assembly coupled to the solar cell array and (ii) a second frame assembly coupled to the first frame assembly arranged to increase the rigidity thereof. The system also has an actuator for rotating the central support and the support frame as well as pivoting the support frame so as to adjust its angle with respect to the earth&#39;s surface, so that the solar cell array is maintained substantially orthogonal to the rays from the sun as the sun traverses the sky.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of and claimspriority to U.S. application Ser. No. 11/830,636, filed on Jul. 30,2007, now U.S. Pat. No. 7,381,886, which is incorporated herein byreference. This application is related to co-pending U.S. applicationSer. No. 12/024,489 filed Feb. 1, 2008, which is a divisional of U.S.application Ser. No. 11/830,636.

This application is also related to co-pending U.S. patent applicationSer. Nos. 11/500,053 filed Aug. 7, 2006, and U.S. patent applicationSer. No. 11/849,033 filed on Aug. 31, 2007 by the common assignee.

BACKGROUND

This disclosure relates generally to a terrestrial solar power systemfor the conversion of sunlight into electrical energy, and, moreparticularly to a solar cell array using IV-V compound semiconductorsolar cells for unitary movement to track the sun. Compoundsemiconductor solar cells, based on III-V compounds, have 28% efficiencyin normal operating conditions. Moreover, concentrating solar energyonto a III-V compound semiconductor photovoltaic cell can increase thecell's efficiency to over 37%. Aspects of a solar cell system includethe specification of the number of cells used to make up an array, andthe shape, aspect ratio, and configuration of the array.

One aspect of a solar cell system is the physical structure of thesemiconductor material layers constituting the solar cell. Solar cellsare often fabricated in vertical, multijunction structures to utilizematerials with different bandgaps and convert as much of the solarspectrum as possible. One type of multijunction structure is the triplejunction solar cell structure consisting of a germanium bottom cell, agallium arsenide (GaAs) middle cell, and an indium gallium phosphide(InGaP) top cell.

In the design of both silicon and III-V compound semiconductor solarcells, one electrical contact is typically placed on a light absorbingor front side of the solar cell and a second contact is placed on theback side of the cell. A photoactive semiconductor is disposed on alight-absorbing side of the substrate and includes one or more p-njunctions, which creates electron flow as light is absorbed within thecell. Grid lines extend over the top surface of the cell to capture thiselectron flow which then connect into the front contact or bonding pad.

The individual solar cells are typically disposed in horizontal arrays,with the individual solar cells connected together in electrical series.The shape and structure of an array, as well as the number of cells itcontains, and the sequence of electrical connections between cells aredetermined in part by the desired output voltage and current of thesystem.

Another aspect of terrestrial solar power systems is the use of lightbeam concentrators (such as lenses and mirrors) to focus the incomingsunrays onto the surface of a solar cell or solar cell array. Thegeometric design of such systems also requires an appropriate solartracking mechanism, which allows the plane of the solar cells tocontinuously face the sun as the sun traverses the sky during the day,thereby optimizing the amount of sunlight impinging upon the cell.

Accurate solar tracking is advantageous because the amount of powergenerated by a given solar cell is related to the amount of sunlightthat impinges on it. In an array, therefore, it is advantageous tooptimize the amount of sunlight that impinges on each constituent solarcell. For example, misalignment of about one degree can appreciablyreduce efficiency. Because arrays are often mounted outdoors and arelarge, heavy structures, this presents challenges. Even moderate windcan cause bending and the array can bend under its own weight. Theseproblems are usually most pronounced in regions near the perimeter ofthe array. As a result, the solar cells that are disposed in the regionswhere bending occurs can become misaligned with the sun, compromisingpower generation.

SUMMARY

The invention relates to a concentrator photovoltaic solar cell arraysystem for producing energy from the sun using one or more sun-trackingsolar cell arrays.

In some implementations, the system includes a central support mountableon a surface and a solar cell array including triple junction III-Vsemiconductor compound solar cell receivers and a support frame coupledto the solar cell array and carried by, and rotatable with respect to,the central support about an axis orthogonal to the central longitudinalaxis. The support frame can include (i) a first frame assembly coupledto the solar cell array and (ii) a second frame assembly coupled to thefirst frame assembly (e.g., including a truss) arranged to increase therigidity of the first frame assembly. The system also has an actuatorfor rotating the central support and the support frame so that the solarcell array is maintained substantially orthogonal to the rays from thesun as the sun traverses the sky. The actuator also can pivot thesupport frame so as to adjust its angle with respect to the earth'ssurface.

Some implementations provide one or more of the following advantages.For example, the system can provide an improved solar cell arrayutilizing a III-V compound semiconductor multijunction solar cells forterrestrial power applications. Some implementations provide a solarcell array for producing approximately 25 kW peak DC power on fullillumination. Some implementations provide the second frame assemblyaligned along the greatest perpendicular dimension (e.g., along thehorizontal axis) of the solar cell array. Some implementations provide atruss coupled to the first frame assembly including a lower chord, anupper chord substantially parallel to the lower chord, two or moresubstantially parallel brace chords coupled to the upper and lowerchords, and at least one diagonal chord disposed between two bracechords and coupled to the upper and lower chords. Some implementationsprovide a truss coupled to the first frame assembly comprising a lowerchord, an upper chord substantially parallel to the lower chord, and atleast one diagonal chord coupled to the lower chord and upper chord.Some implementations provide a plurality of series-connected receiverseach with a III-V semiconductor solar cell in a Fresnel lens based solarconcentrator subarray for terrestrial power applications. Someimplementations provide a lower chord including at least a portion ofthe first frame assembly. Some implementations provide a first frameassembly including a generally rectangular frame member comprising upperand lower parallel members oriented in a direction substantiallyparallel to the surface to which the center support is mountable,wherein the upper chord is coupled to the lower parallel member by atleast one truss support member. Some implementations provide bracechords arranged substantially orthogonal to a plane defined by the solarcell array. Some implementations provide that the direction of theperpendicular distance from the lower chord to the upper chord issubstantially orthogonal to a plane defined by the solar cell array.Some implementations provide that the width of the lower chord issubstantially the same as the width of the solar cell array, wherein thewidth of the solar cell array is measured in a direction substantiallyparallel to the surface to which the central support is mountable. Someimplementations provide that the width of the first frame assembly andthe width of solar cell array are substantially the same, wherein thewidth of the solar cell array is measured in a direction substantiallyparallel to the surface to which the central support is mountable. Someimplementations provide that the truss is arranged in a directionorthogonal to a plane defined by the first frame assembly. Someimplementations provide a solar cell array including a plurality ofsolar cell modules, each module including a plurality of Fresnel lenseswherein each Fresnel lens is disposed over a single solar cell forconcentrating by a factor in excess of 500× the incoming sunlight ontothe solar cell and producing in excess of 13 watts of DC power at AM 1.5solar irradiation per solar cell with conversion efficiency in excess of37%. Some implementations provide a solar cell array including aplurality of solar cells and a corresponding plurality of Fresnel lenseseach of which is disposed over a single solar cell for concentrating bya factor in excess of 500× the incoming sunlight onto the solar cell andproducing in excess of 13 watts of DC power at AM 1.5 solar irradiationper solar cell with conversion efficiency in excess of 37%. Someimplementations provide a truss mounted above the vertical center (i.e.,above the center of its height) of the solar cell array. Someimplementations provide a central support constituted by a first memberprovided with means for mounting the central support on the surface, anda second member rotatably supported by, and extending upwardly from, thefirst member. Some implementations provide the advantage that thesupport frame is mounted on a cross member which is rotatably mountedwith respect to the second member of the central support about an axisorthogonal to the central longitudinal axis. Some implementationsprovide a first frame assembly including a generally rectangular framemember having a plurality of parallel support struts that are parallelto the shorter sides of the rectangular frame member. Someimplementations provide a first frame assembly further includes aplurality of oblique support struts. Some implementations provide thatthe truss prevents a deflection greater than 1 degree near the perimeterof the solar cell array. Some implementations provide an array of III-Vsemiconductor solar cell concentrator modules with a solar tracker forterrestrial power applications. Some implementations provide aterrestrial solar power system constituted by a plurality of solar cellarrays each mounted on a post to track the sun, wherein each array issized and spaced apart from each other over the ground so as to maximizethe number of cells that can be implemented over a given ground area.Some implementations provide a solar cell array system in which a singlesolar tracking tower produces 25 kW of peak DC power for terrestrialpower applications.

Other features and advantages will be readily apparent from the detaileddescription, accompanying drawings and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of an implementation of a terrestrialsolar cell system.

FIG. 1B is a second perspective view of the implementation of FIG. 1A.

FIG. 1C is a perspective view of an implementation of a terrestrialsolar cell system.

FIG. 1D is a perspective view of an implementation of a support framefor use with the terrestrial solar cell system of FIG. 1C.

FIG. 1E is a simplified side view of an implementation of a terrestrialsolar cell system.

FIG. 1F is a side view of an implementation of a terrestrial solar cellsystem.

FIG. 2 is a perspective view of the solar cell system implementation ofFIG. 1A viewed from the opposite side thereof.

FIG. 3 is a perspective view of a portion of an implementation of asolar cell subarray utilized in a terrestrial solar cell system.

FIG. 4 is a perspective view of an implementation of a solar cellreceiver utilized in a solar cell subarray.

FIG. 5 is a top plan view of a single solar cell subarray.

Additional advantages and features will become apparent to those skilledin the art from this disclosure, including the following detaileddescription. While the invention is described below with reference toimplementations thereof, it should be understood that the invention isnot limited to those implementations. Those of ordinary skill in the arthaving access to the teachings herein will recognize additionalapplications, modifications and implementations in other fields, whichare within the scope of the invention as disclosed and claimed hereinand with respect to which the invention could be of utility.

DETAILED DESCRIPTION Overview

A terrestrial solar power system converts sunlight into electricalenergy utilizing, e.g., multiple mounted arrays spaced in a grid overthe ground. The array of solar cells has a particular optical size andaspect ratio (e.g., between 1:3 and 1:5), and is mounted for unitarymovement on a cross-arm of a vertical support that tracks the sun. Thearray can include subarrays, sections, modules and/or panels.

The solar tracking mechanism allows the plane of the solar cells tocontinuously face the sun as the sun traverses the sky during the day,thereby optimizing the amount of sunlight impinging upon the cells. Theamount of power generated by the array is directly related to the amountof sunlight impinging upon the constituent solar cells. Since a givenarray can comprise many (e.g., a thousand or more) solar cells, it isadvantageous to maintain the solar alignment of the entire array. This,however, is difficult in practice because it is not uncommon for anarray to be upwards of 18 meters wide (about 59 feet) and 7.5 metershigh (about 25 feet). Given the size of the array, solar cells near theperimeter may become misaligned due to bending or flexing of the array.Bending or flexing can arise, e.g., as a result of wind or the weight ofthe array causing the structure to bend. Since misalignment as little asone degree or less is detrimental in some implementations, it isdesirable to minimize bending or flexing of the array.

Implementations of a Terrestrial Solar Cell System

An implementation of a terrestrial solar cell system is illustrated inFIG. 1A. In general terms, the system comprises three major components.The first major component is the central support (11 a and 11 b). Thecentral support is mounted to a surface and is capable of rotating aboutits longitudinal axis. Depending on the implementation, the surface canbe, e.g., the ground or a concrete foundation formed in the ground.Disposed on or adjacent to the surface is a drive mechanism 100 (e.g., agearbox) that couples to the central support. The drive mechanism 100enables the inner member 11 b to rotate relative to the outer member 11a, e.g., for moving the solar cell array such that it tracks the sun.

The second major component is the support frame 15. The support frame 15couples to the central support and is adapted to support a solar cellarray (e.g., array 10). The third major component is the solar cellarray 10. The solar cell array 10 includes multiple subarrays or panels16 and is coupled to, and supported by, the support frame 15. The solarcell array 10 converts sunlight into electricity, and normally is keptfacing the sunlight by the rotation of the central support. In thisimplementation, each of the solar cell subarrays 16 is divided intothirteen sections 17. Each section 17 includes a 2×7 panel ofconcentrating lenses (e.g., item 320 of FIG. 3) each lens disposed overa single receiver (e.g., item 19 b of FIGS. 3 and 4). The receiver, aprinted circuit or subassembly, includes a single III-V compoundsemiconductor solar cell together with additional circuitry such as abypass diode (not shown). In some implementations, each section 17 is amodule, e.g., a discrete assembly. In some implementations, the sections17 are separated from each other by perforated dividers.

In the illustrated implementation, the central support includes an outermember 11 a and an inner member 11 b. The outer member 11 a isconnectable to a support mounted on the surface by bolts. The innermember 11 b is rotatably mounted within the member 11 a and supports across member 14 which is connected to a support frame 15. The supportframe 15 also is supported on the inner member 11 b by a pair ofinclined arms 14 a which extend respectively from two of the supportstruts 150 b (visible in FIG. 1B) to the base of the inner member 11 b.The inclined arms 14 a are coupled to each other by a cross-member 14 b(see also FIG. 1B) that increases their structural integrity. Themounting of the support frame 15 in this manner ensures that it is fixedto the inner member 11 b of the central support in such a manner that itis rotatable about its central longitudinal axis through members 11 aand 11 b.

The support frame 15 has a rectangular frame 15 a and a truss 15 b. Therectangular frame 15 a includes two shorter members (see items 15 a 3and 15 a 4 of FIG. 1B) that are oriented in a direction parallel to theheight (see dimension “C” of FIG. 1B) of the solar cell array 10 and twolonger members (see items 15 a 1 and 15 a 2 of FIG. 1B) that areoriented in a direction parallel to the width (see dimension “A” of FIG.1B) of the solar cell array 10. In this implementation, the width of therectangular frame 15 a is approximately equal to the width of the solarcell array 10. Although this configuration can result in improvedrigidity (e.g., less bending of the solar cell array 10 near itsperimeter), it is not required. For example, to reduce material cost,the width of the rectangular frame 15 a can be reduced.

The truss 15 b is coupled to the rectangular frame 15 a in a manner thatincreases the rigidity of the rectangular frame 15 a, and thus, therigidity of the solar cell array 10. The truss, therefore, improvesalignment of the constituent solar cells (particularly those near theperimeter) such that power generation is substantially improved. Thetruss 15 b can function to prevent deflection greater than 1 degree nearthe perimeter of the solar cell array 10. In some implementations, thetruss 15 b is aligned with

In this implementation, the truss 15 b includes a lower truss chord 152d, an upper truss chord 152 c, parallel truss brace chords 152 b anddiagonal truss chords 152 a. The parallel truss brace chords 152 b anddiagonal truss chords 152 a are coupled between the upper and lowertruss chords 152 c and 152 d. The parallel truss brace chords 152 b areoriented substantially parallel to one another and perpendicular to theupper and lower truss chords 152 c and 152 d. The particularconfiguration of chords 152 a-d can vary with the implementation. Forexample, truss 15 b may include no diagonal truss chords (e.g., aVierendeel truss), no parallel truss brace chords (e.g., a latticetruss), or the relative orientation of the diagonal truss chords canvary (e.g., a Pratt truss or a Howe truss).

In this implementation, the truss 15 b is coupled to the rectangularframe 15 a by truss support members 151 a. Also, in this implementationthe rectangular frame 15 a and truss 15 b are integrated, i.e., thelower truss chord 152 d comprises one of the longer members of therectangular frame 15 a. In this implementation, the width of the truss15 b (e.g., the width of the lower chord 152 d) is approximately equalto the width of the solar cell array 10 and the rectangular frame 15 a.Although this configuration can result in improved rigidity (e.g., lessbending of the solar cell array 10 near its perimeter), it is notrequired. For example, to reduce material cost, the width of the truss15 b can be reduced.

In this implementation, the truss 15 b is arranged such that thedirection of its height (i.e., the perpendicular direction between thelower truss chord 152 d and the upper truss chord 152 c) issubstantially orthogonal to the plane defined by the height and width ofthe solar cell array 10. Although this configuration can result inimproved rigidity, it is not required. For example, to accommodatepackaging requirements, the truss 15 b can be coupled such that thedirection of its height is not substantially orthogonal to the planedefined by the height and width of the solar cell array 10.

In the illustrated implementation, the truss 15 b is not disposed in thevertical center (i.e., along dimension “C” of FIG. 1B) of the solar cellarray 10. The inventors discovered that placing the truss 15 b above thevertical centerline of the solar cell array 10 can result in improvedmaneuverability with respect to the center support. As a result, thecentral support can move the solar cell array 10 to track sunlightwithout interference by the presence of the truss 15 b.

Although the illustrated implementation utilizes a truss 15 b toincrease the rigidity of the rectangular frame 15 a, other structuresare possible. For example, a solid plate can be used. In anotherexample, a solid plate having one or more cutouts can be used. Moreover,a very simple truss can be used that omits chords 152 a and 152 b infavor of simply coupling upper truss chord 152 c to the lower trusschord 152 d. Such a truss can include one or more additional membersthat are oriented parallel to the upper truss chord 152 c.

FIG. 1B is a rear-facing view of the terrestrial solar cell system ofFIG. 1A, with the solar cell array 10 oriented orthogonally to thesurface to which the central support is mounted (e.g., the ground). Asillustrated, the truss 15 b aligned along the greatest perpendiculardimension (i.e., along dimension “A”) of the array 10. This isadvantageous because the array is generally more prone to bending alonga longer axis than along a shorter axis (e.g., along dimension “C”). Inthis implementation, dimension “A”, the width of the solar cell array10, is approximately 18.1 meters (approximately 59.4 feet), dimension“B”, the width of subarray 16, is approximately 1.8 meters(approximately 5.9 feet) and dimension “C”, the height of the solar cellarray 16, is approximately 7.5 meters (approximately 24.6 feet). Such animplementation has a solar collecting area of approximately 98.95 squaremeters (approximately 1065.1 square feet) and weighs approximately10,191 kilograms (approximately 10.03 tons). If constructed in a mannerconsistent with this disclosure, such an implementation can have a windsurvival rating of 145 kilometers/hour (approximately 90.1 miles/hour).

In FIG. 1B, the view of the truss 15 b is largely obscured because it isarranged orthogonally to the plane defined by the height and width ofthe solar cell array. However, this view illustrates truss supportmembers 151 a, which couple the truss 15 b to the rectangular frame 15a. In particular, the truss support members 151 couple to a long member15 a 1 or 15 a 2 of the rectangular frame 15 a (in this implementation,the lower long member 15 a 2) and the upper truss chord 152 c (see FIG.1A). In this implementation, four truss support members 151 a are shownarranged diagonally. While arranging the truss support members 151 adiagonally offers the advantage of resisting tension and compression, itis not necessary. Also, more or fewer truss support members 151 a can beemployed depending on the implementation.

This view also reveals additional features of the rectangular frame 15a. To improve the structural integrity of the rectangular frame, severalcross members 150 a couple the upper long member 15 a 1 to the lowerlonger member 15 a 2. The cross members 150 a are complemented byparallel members 150 b (which, in this implementation, are orientedsubstantially parallel to the shorter members 15 a 3 and 15 a 4). Two ofthe parallel members 150 b serve the additional purpose of providing amounting point to which the cross member 14 couples.

This view again illustrates that the width of the rectangular frame 15 ais approximately the same width as the solar cell array 10 (i.e., it isabout 18.1 meters wide). This view also illustrates that the location ofthe truss 15 b is above the centerline of dimension C.

FIG. 1C illustrates an implementation of a terrestrial solar cell systemwith the plane defined by the height and width of the solar cell array10 oriented parallel to the surface to which the central support ismounted (e.g., the ground). This implementation utilizes a truss 15 b′having a configuration slightly different than that of 15 b. This truss15 b′ omits parallel truss brace chords 152 b in favor of using alldiagonal truss chords 152 a. FIG. 1D illustrates a perspective view of asupport frame 15 comprising truss 15 b′.

FIG. 1E is simplified view of a terrestrial solar cell system, viewedfrom a direction orthogonal to the plane defined by the height and widthof the solar cell array 10. As illustrated, the truss (15 b or 15 b′depending on the implementation) is disposed above the centerline ofdimension C. Also, the truss (15 b or 15 b′) in this implementation isoriented at a right angle (θ) relative to the solar cell array 10.

FIG. 1F is a side view of an implementation of a terrestrial solar cellsystem, viewed from a direction orthogonal to the plane defined by theheight and width of the solar cell array 10. As illustrated, the truss(15 b or 15 b′ depending on the implementation) is disposed above thecenterline of dimension C. By locating the truss above the verticalcenter of the solar cell array, the truss does not obstruct movement ofthe array relative to the central support (11 a, 11 b). Jackscrew 111and mating threaded rod 112 together can adjust the angle of the array10 through at least a portion of the range indicated by path 113. Thus,the jackscrew 111 (e.g., in combination with a drive mechanism such asitem 100 of FIG. 1A) enables pivoting the support frame 15, and thus thearray 10, so as to adjust its angle with respect to the earth's surface

FIG. 2 is a perspective view of the solar cell system implementation ofFIG. 1A viewed from the opposite side thereof. This perspectiveillustrates the division of each subarray 16 into sections 17. Eachsection 17 includes a base 18, which provides a structural foundationfor each receiver 19 (see FIGS. 3 and 4). In some implementations, thereis one base 18 per subarray 16, shared by each constituent section 17.In some implementations, the base 18 is structurally distinct for eachsection 17.

FIG. 3 is a cutaway view of a solar cell subarray 16 depicting onesection 17 on base 18. In this implementation, section 17 includes asheet 320 including a 2×7 matrix of Fresnel lenses (20 a-20 j areshown), a 2×7 matrix of secondary optical elements (“SOE”, an example ofwhich is shown as item 201) and a 2×7 matrix solar cell receivers 19(fourteen are shown, including items 19 a-19 j). In someimplementations, the sheet 320 is an integral plastic panel and eachFresnel lens (e.g., items 20 a-20 j) is a nine-inch square. In theillustrated implementation, each Fresnel lens (e.g., 20 b) and itsassociated receiver (e.g., 19 b) and SOE (e.g., 201) are aligned suchthat the light concentrated by the lens is optimally received by thesolar cell of the associated receiver. In the illustratedimplementation, section 17 is delineated from the remainder of the base18 by a divider 301 (which can be perforated). The base 18 also whichserves to dissipate heat from the receivers, and more particularly fromthe individual solar cells.

FIG. 4 illustrates a receiver 19 b in more detail. The receiver 19 b hasa plate 203, a printed circuit board (“PCB”) 204, an SOE 201 and a mount202. The plate 203 couples the receiver 19 b to the base 18 (see FIGS. 2and 3). In some implementations, the plate 203 is constructed of amaterial having a high thermal conductivity such that the heat from thePCB 204 (which includes, for example, a solar cell and a bypass diode)is dissipated away efficiently. In some implementations, the plate 203is made of aluminum. In some implementations, the PCB 204 includes aceramic board with printed electrical traces.

The mount 202, which is coupled to the plate 203 at two positions, formsa bridge that aligns the SOE 201 with the solar cell of the PCB 204. TheSOE 201 gathers the light from its associated lens 20 and focuses itinto the solar cell on the PCB 204. In some implementations, each solarcell receiver 19 is provided with a corresponding SOE 201. The SOE 201includes an optical inlet 201 a and optical outlet (facing the PCB 204)and a body 201 b. The SOE 201 is mounted such that the optical outlet isdisposed above the solar cell of the PCB 204. Although it can varydepending on the implementation, the SOE 201 in the illustrated exampleis mounted such that the optical outlet is about 0.5 millimeters fromthe solar cell. The SOE 201 (including the body 201 b) can be made ofmetal, plastic, or glass or other materials.

In some implementations, the SOE 201 has a generally square crosssection that tapers from the inlet 201 a to the outlet. The insidesurface 201 c of the SOE reflects light downward toward the outlet. Theinside surface 201 c is, in some implementations, coated with silver oranother material for high reflectivity. In some cases, the reflectivecoating is protected by a passivation coating such as SiO2 to protectagainst oxidation, tarnish or corrosion. The path from the optical inlet201 a to the optical outlet forms a tapered optical channel that catchessolar energy from the corresponding lens 20 and guides it to the solarcell. As shown in this implementation, the SOE 201 has four reflectivewalls. In other implementations, different shapes (e.g., three-sided toform a triangular cross-section) may be employed.

In some cases, the corresponding lens 20 does not focus light onto aspot that is of the dimensions of the solar cell or the solar trackingsystem may not perfectly point to the sun. In these situations, somelight does not reach the solar cell. The reflective surface 201 cdirects light to the solar cell 30. The SOE also can homogenize (e.g.,mix) light. In some cases, it also has some concentration effect.

In some implementations, the optical inlet 201 a is square-shaped and isabout 49.60 mm×49.60 mm, the optical outlet is square-shaped and isabout 9.9 mm×9.9 mm and the height of the optical element is about70.104 mm. These dimensions can vary with the design of the solar cellmodule, section and/or the receiver. For example, in someimplementations the dimensions of the optical outlet are approximatelythe same as the dimensions of the solar cell. For an SOE having thesedimensions, the half inclination angle is 15.8 degrees.

In a particular implementation, as illustrated in the plan view of FIG.5, the subarray 16 is about 7.5 meters high (y direction) and 1.8 meterswide (x direction) and includes sections 17 each having a 2×7 matrix ofFresnel lenses 20 and receivers 19 (see FIGS. 3 and 4). Each receiver 19produces over 13 watts of DC power on full AM 1.5 solar irradiation. Thereceivers are connected by electrical cables in parallel or in series sothat the aggregate 182 receivers in an entire subarray 16 can produce inexcess of 2500 watts of peak DC power. Each of the subarrays is in turnconnected in series, so that a typical array (e.g., item 10) can producein excess of 25 kW of power.

A motor provides drive to rotate the member 11 b relative to the member11 a, and another motor provides drive to rotate the cross member 14(and hence the support frame 15) relative to the central support 11about its longitudinal axis. Control means are provided (e.g., disposedin drive mechanism 100 of FIG. 1) for controlling rotation of the member11 b relative to the member 11 a, and for controlling rotation of thecross member 14 (and the support frame 15) about its axis to ensure thatthe planar exterior surface of each of the sections 17 comprisingFresnel lenses 20 is orthogonal to the sun's rays. In someimplementations, the control means is a computer controlled machine,using software that controls the motors in dependence upon the azimuthand elevation of the sun relative to the system. In someimplementations, each of the Fresnel lenses 20 concentrates incomingsunlight onto the solar cell in an associated receiver (e.g., item 19 b)by a factor of over 500X, thereby enhancing the conversion of sunlightinto electricity with a conversion efficiency of over 37%. In someimplementations, the concentration is 520×.

In some implementations, the system is refractive and uses an acrylicFresnel lens to achieve 520× concentration with an f# of approximately2. A reflective secondary optical element can be used, as described inconnection with FIG. 4. An acceptance angle for an individualcell/optics system is +/−1.0 degrees. The efficiency of the opticalsystem on-sun is 90% with the acceptance angle defined at a point wherethe system efficiency is reduced by no more than 10% from its maximum.Some implementations, however, may define a different acceptance angle,e.g. +/−0.1 degrees. In some implementations, each solar cell isassembled in a ceramic package that includes a bypass diode and a twospaced-apart connectors. In some implementations, 182 cells areconfigured in a subarray. The number of cells in a subarray arespecified so that at maximum illumination, the voltages added togetherdo not exceed the operational specifications of the inverter.

Additional details of an example of the design of the receiver aredescribed in U.S. patent application Ser. No. 11/849,033 filed Aug. 31,2007 and herein incorporated by reference.

Additional details of an example of the design of the semiconductorstructure of the triple junction III-V compound semiconductor solar cellreceiver (e.g., item 19) are described in U.S. application Ser. No.12/020,283, filed Jan. 25, 2008 herein incorporated by reference.

In the illustrated example, the solar cell is a triple junction device,with the top junction based on InGaP, the middle junction based on GaAs,and the bottom junction based on Ge. Typical band-gaps for the cell are1.9 eV/1.4 eV/0.7 eV, respectively. Typical cell performance as afunction of temperature indicate that Voc changes at a rate of −5.9 mV/Cand, with respect to temperature coefficient, the cell efficiencychanges by −0.06%/C absolute.

One electrical contact is typically placed on a light absorbing or frontside of the solar cell, and a second contact is placed on the back sideof the cell. A photoactive semiconductor is disposed on alight-absorbing side of the substrate and includes one or more p-njunctions, which creates electron flow as light is absorbed within thecell. Grid lines extend over the top surface of the cell to capture thiselectron flow which then connect into the front contact or bonding pad.It is advantageous to maximize the number of grid lines over the topsurface of the cell to increase the current capacity without adverselyinterfering with light transmission into the active semiconductor area.

While implementations have been illustrated and described as embodied ina solar cell array using III-V compound semiconductors, it is notintended to be limited to the details shown, since various modificationsand structural changes may be made without departing in any way from thespirit of the present invention. Accordingly, other implementations arewithin the scope of the claims.

1. A concentrator photovoltaic solar cell array system for producingenergy from the sun using one or more sun-tracking solar cell arrays,the system comprising: a central support mountable on a surface, androtatable about its central longitudinal axis; a substantially planarsolar cell array including a plurality of triple junction III-Vsemiconductor compound solar cell receivers; a support frame coupled tothe solar cell array and carried by and rotatable with respect to thecentral support about an axis orthogonal to the central longitudinalaxis, the support frame comprising: a first frame assembly coupled tothe solar cell array; and a second frame assembly coupled to the firstframe assembly arranged to increase the rigidity of the first frameassembly; and an actuator for rotating the central support and thesupport frame so that the solar cell array is maintained substantiallyorthogonal to the rays from the sun as the sun traverses the sky.
 2. Asystem as claimed in claim 1 wherein the second frame assembly comprisesa truss.
 3. A system as claimed in claim 1 wherein the second frameassembly comprises two shorter beams and one longer beam, wherein afirst end of each shorter beam is coupled to the first frame assemblyand a second end of each shorter beam is coupled to the longer beam. 4.A system as claimed in claim 1 wherein the second frame assembly isarranged along the greatest perpendicular dimension of the solar cellarray.
 5. A system as claimed in claim 2 wherein the truss is arrangedalong the greatest perpendicular dimension of the solar cell array.
 6. Asystem as claimed in claim 2 wherein the truss comprises a lower chord,an upper chord substantially parallel to the lower chord, two or moresubstantially parallel brace chords coupled to the upper and lowerchords, and at least one diagonal chord disposed between two bracechords and coupled to the upper and lower chords.
 7. A system as claimedin claim 2 wherein the truss comprises a lower chord, an upper chordsubstantially parallel to the lower chord, and at least one diagonalchord coupled to the lower chord and upper chord.
 8. A system as claimedin claims 6 or 7 wherein the lower chord comprises at least a portion ofthe first frame assembly.
 9. A system as claimed in claims 6 or 7wherein the first frame assembly comprises a generally rectangular framemember comprising upper and lower parallel members oriented in adirection substantially parallel to the surface to which the centersupport is mountable, wherein the upper chord is coupled to the lowerparallel member by at least one truss support member.
 10. A system asclaimed in claim 6 wherein the brace chords are arranged substantiallyorthogonal to a plane defined by the solar cell array.
 11. As system asclaimed in claim 7 wherein the direction of the perpendicular distancefrom the lower chord to the upper chord is substantially orthogonal to aplane defined by the solar cell array.
 12. A system as claimed in claims6 or 7 wherein the width of the lower chord is substantially the same asthe width of the solar cell array, wherein the width of the solar cellarray is measured in a direction substantially parallel to the surfaceto which the central support is mountable.
 13. A system as claimed inclaim 1 wherein the second frame assembly is arranged in a directionorthogonal to a plane defined by the first frame assembly.
 14. A systemas claimed in claim 2 wherein the truss is arranged in a directionorthogonal to a plane defined by the first frame assembly.
 15. A systemas claimed in claim 1 wherein the second frame assembly is arranged in adirection orthogonal to a plane defined by the solar cell array.
 16. Asystem as claimed in claim 2 wherein the truss is arranged in adirection orthogonal to a plane defined by the solar cell array.
 17. Asystem as claimed in claim 1 wherein the width of the first frameassembly and the width of solar cell array are substantially the same,wherein the width of the solar cell array is measured in a directionsubstantially parallel to the surface to which the central support ismountable.
 18. A system as claimed in claim 2 wherein the truss ismounted above the vertical center of the solar cell array.
 19. A systemas claimed in claim 1, wherein the solar cell array comprises aplurality of solar cells and a corresponding plurality of Fresnel lenseseach of which is disposed over a single solar cell for concentrating bya factor in excess of 500× the incoming sunlight onto the solar cell andproducing in excess of 13 watts of DC power at AM 1.5 solar irradiationper solar cell with conversion efficiency in excess of 37%.
 20. A systemas claimed in claim 1, wherein the central support is constituted by afirst member provided with means for mounting the central support on thesurface, and a second member rotatably supported by, and extendingupwardly from, the first member.
 21. A system as claimed in claim 1,wherein the support frame is mounted on a cross member which isrotatably mounted with respect to the second member of the centralsupport about an axis orthogonal to the central longitudinal axis.
 22. Asystem as claimed in claim 1, wherein the first frame assembly comprisesa generally rectangular frame member having a plurality of parallelsupport struts that are parallel to the shorter sides of the rectangularframe member.
 23. A system as claimed in claim 22, wherein the firstframe assembly further comprises a plurality of oblique support struts.24. A system as claimed in claim 1 wherein the solar cell array isarranged to produce approximately 25 kW peak DC power on fullillumination.
 25. A system as claimed in claim 1 comprising at least oneinclined strut coupled to the central support and the first frameassembly.
 26. A concentrator photovoltaic solar cell array system forproducing energy from the sun using a plurality of sun-tracking solarcell arrays, comprising: a central support mountable on a surface androtatable about its central longitudinal axis; a rectangular solar cellarray having an aspect ratio between 1:3 and 1:5, with the longitudinalof the array substantially parallel to the surface for producing inexcess of 25 kW peak DC power on full illumination and including aplurality of triple junction III-V semiconductor compound solar cellreceivers mounted on the support frame; and a support frame coupled tothe solar cell array and carried by and rotatable with respect to thecentral support about an axis orthogonal to the central longitudinalaxis, the support frame comprising: a first frame assembly coupled tothe solar cell array; and means for increasing the rigidity of the firstframe assembly; and an actuator for rotating the central support and thesupport frame so that the solar cell array is maintained substantiallyorthogonal to the rays from the sun as the sun traverses the sky. 27.The system of claim 26 wherein the means for increasing the rigidity ofthe first frame assembly prevents a deflection greater than 1 degreenear the perimeter of the solar cell array.
 28. A concentratorphotovoltaic solar cell array system for producing energy from the sunusing one or more sun-tracking solar cell arrays, the system comprising:a central support mountable on the ground, and being capable of rotationabout its central longitudinal axis; a rectangular solar cell array forproducing in excess of 25 kW peak DC power on full illuminationincluding a plurality of triple junction III-V semiconductor compoundsolar cell receivers; a support frame coupled to solar cell array andcarried by and rotatable with respect to the central support about anaxis orthogonal to the central longitudinal axis, the support framecomprising: a rectangular frame assembly coupled to the solar cell arraycomprising first and second long members aligned in a directionsubstantially parallel to the direction of the largest perpendiculardimension of the solar cell array and comprising first and second shortmembers aligned in a direction substantially perpendicular to thedirection of the largest perpendicular dimension of the solar cellarray; and a truss assembly coupled to at least one of the first andsecond long members, the truss assembly comprising a lower chord, anupper chord substantially parallel to the lower chord, and at least onediagonal chord coupled to the lower chord and upper chord; and anactuator for rotating the central support and the support frame so thatthe solar cell array is maintained substantially orthogonal to the raysfrom the sun as the sun traverses the sky.
 29. The system of claim 28wherein the lower chord comprises one of the first and second longmembers.
 30. The system of claim 28 wherein the direction of theperpendicular distance from the lower chord to the upper chord issubstantially orthogonal to a plane defined by the solar cell array. 31.The system of claim 28 wherein the width of the rectangular frameassembly is approximately the same as the largest perpendiculardimension of the solar cell array.
 32. The system of claim 28 whereinthe truss assembly is located above the vertical center of the solarcell array.
 33. The system of claim 28 wherein the first and second longmembers and first and second short members are formed as a unitarystructure.